The invention relates to electro-acoustic transducers and, in particular, to transducers selectively capable of responding either to transducer displacement or to pressure.
As will become apparent, the present transducer primarily is intended for use as an underwater reception sensor of hydrophone. However, its principle of operation as well as its general component arrangement readily adapts for other uses.
As is known, practically all underwater reception sensors have been of the type which respond to pressure variations as contrasted with transducer displacement and one reason for the pressure preference has been the relative ease of constructing pressure units such as those formed of the familiar piezo-electric materials. Displacement-type sensors, nevertheless, have high sensitivity and, as will be demonstrated, can be constructed economically and with relative ease. Consequently, they can be used advantageously in conjunction with or as substitutes for the widely-used piezo-electric pressure sensors.
It is also known that pressure sensors of the piezo-electric type or of any other type have certain limitations in their use as underwater sensors. In particular, their response to the so-called infrasonic frequencies may not be reliable because infrasonic wave lengths easily can exceed the water depths in which the pressure sensors are used. The infrasonce region generally includes frequencies of 30 Hz or below. Because of this limitation there are only a few limited observations available on shallow water ambient noise measurements at infrasonic frequencies. Displacement sensors are not subject to the same wave lengths limitations and thus are capable of providing valuable data.
A primary object of the present invention is to provide a sensitive, easily constructed means for sensing displacement as well as pressure-producing forces and, in particular, to provide an underwater reception sensor having these capabilities.
Another object is to provide a capacitive-type underwater reception sensor capable of utilizing electrets as a power source.
Another object is to provide a displacement sensor having a high sensitivity for selected frequency regions including the region of the infrasonic wave length.
Other objects and intended advantages will become more apparent in the ensuing description.